Project Summary. The proposed research involves the development of crosslinked organic filler particles and surface- modified inorganic fillers that are functionalized with radical addition-fragmentation chain transfer (AFT) moieties for stress reduction in dental restorative composites. Although there have been significant advances in the design of functionalized inorganic fillers and addition of AFT monomers to the resin (polymerizable) phase, stress development during polymerization is still a significant issue in dental restoratives that can lead microcracking of the composite, delamination from the tooth, or even tooth fracture. The design of functional, crosslinked AFT-based particles and subsequent implementation into classic dental resins will enable covalent bond rearrangement within the filler particle to relax stress as it develops. Similarly, incorporation of AFT moieties onto the surface of inorganic fillers is an additional method to introduce dynamic chemistry to the filler phase while maintaining the same mechanical properties of the resin. The purposed research will address stress concentration on the filler and filler/resin interface by taking advantage the Covalent Adaptable Networks (CANs) paradigm. This approach replaces the static bonds that normally exist in a crosslinked network with dynamic bonds that rearrange to a lower stress state in the presence of proper activating species. In the case of AFT, the activating species are free radicals, which are also the active species during the photopolymerization of the dimethacrylate based resins used in most dental restorative composites. The first aim is to develop AFT particles that reduce stress development both during polymerization and upon external loading of classic glassy systems, then implement these particles directly as an additive to standard dental resin formulations and study their effect on the stress behavior. Next, we will develop AFT organic/inorganic nanocomposite particles with functionalized silica nanoparticles to circumvent limitations such as resin viscosity and modulus of the composite. The second aim is to functionalize silica nanoparticles with AFT moieties to enable bond exchange only at the filler/matrix interface without maintaining the mechanical properties of the final composite. We will then incorporate the both the organic AFT particles and the functionalized to investigate possible synergistic effects of both particle types in a dental composite. To accomplish these goals, the purposed training plan will enable trainee to interact with mentors and learn from their technical and practical experience with dental materials. The selected mentoring committee has extensive experience with synthesis, characterization of polymeric dental materials, and clinical implementation, and industrial considerations. The trainee will receive relevant and meaningful training in research-oriented areas and conducting research both thoughtfully and ethically.